Trapezoidal-type standing seam roofing systems were first made with exposed fasteners which pierced the pans. This proved to be unacceptable because of leaks at the penetrations. Also, especially with lengthy pans, thermal expansion and contraction would either cause the fasteners to fail or create elongated holes in the pans, thus creating additional sources of leaks as well as weakening the structure.
To combat these problems, sealed clips were developed. Conventional clips were of two piece construction. The upper portions were folded or interlocked at the uppermost section of the standing seam while the lower section was fastened to the structure. Usually, the two pieces were connected with a slot to allow for thermal movement.
Although the newer designs seemed to address the problems of the exposed, pan-penetrating fasteners, they were still subject to failure, primarily of two types. The first type of failure occurred when the roofing system would pull off from the support structure during negative (uplift) wind conditions. The second type of failure was leaking.
Conventional systems were tested in laboratories using steady-state conditions; such tests did not, however, mimic actual use conditions. Winds are not constant so that uplift forces fluctuate. These fluctuating conditions, which flex the metal back and forth, can work the clip loose or fatigue it to failure. When the uplift conditions are severe, the roofing panel deforms upward in a convex condition; this tends to pull the standing seams and ribs apart. Conventional two piece clips hold standing seam roofing panels near the tops of their joints, thus allowing flexing of the joints. Flexing not only can ruin the weatherproof seal of the standing seam joint, but can also cause the legs of the seam to spread apart (in what is called rib spread) which can lead to disengagement of the clips from the pans and thus failure of the roofing system.
Another cause of clip failure originates at installation. Conventional two part standing seam clips are slidably mounted to one another to accommodate thermal movement. During installation it becomes difficult, if not impossible, to determine what the proper relative positions between the two pieces of the clip should be when installed. That is, should the clips be positioned at one sliding extreme, the other sliding extreme or centered? In practice, the clips are installed centered, even though that may not be the appropriate place. Because of this, the clips may reach their limits of relative movement during the daily thermal expansion and contraction of the pans. Subjecting the clips through this cyclic stress can result in clip or fastener failure.
Another problem with conventional standing seam joints is created by leaks at the terminations of the standing seams, such as at eaves, penetrations, hips, ridges, and so forth. The primary cause of leaks is the inability to seal the end of the standing seam. This is primarily due to its accordion nature. The ends of panels creating the standing seam joint are highly vulnerable to distortion. The open ends of standing seam joints seldom fit the preformed neoprene and metal closures which are designed for use with undamaged, properly shaped standing seam joints. Despite attempts to fill any voids with sealants, leaks often occur through gravity penetration, wind forced penetration and capillary action.